The present invention is directed toward the field of radio frequency (RF) or microwave signal-carrying circuits. In particular, the invention discloses a printed lumped element stripline architecture for implementing various RF passive circuits. The application describes in detail two exemplary circuits, a -3 dB three-port coupler and a 90.degree. branch line four-port coupler, although the invention can be extended to any type of passive circuit. The invention also discloses methods of making the stripline architecture that includes the printed circuits. The novel stripline structure provides a ground-signal-ground multi-layer architecture in which the signal carrying circuit is printed on a first dielectric substrate, and additional dielectric substrates are laminated on opposite sides of the first substrate, each additional substrate having a ground plane on a side opposite the printed circuit side of the first substrate. By printing lumped elements to form the desired circuit within a multi-layer stripline architecture, the present invention overcomes many disadvantages associated with prior and current circuit packaging techniques.
The present invention finds its greatest utility in space-based communication satellites. However, the techniques and circuit architecture disclosed herein are not limited to use in communication satellites and can be extended to other applications. Satellites typically employ many signal carrying circuits, such as power splitters/combiners (i.e. couplers), switch matrices, local oscillator distribution networks, and corporate feed networks for phased arrays, to name a few. Each of these circuits may include several passive components, such as resistors, inductors and capacitors. Reducing the size, mass, cost and electromagnetic radiation associated with these circuits is of critical importance to the satellite industry and requires careful design of the circuit structure and packaging.
Present methods of making RF/Microwave circuits for the satellite industry include the following techniques: (1) printing distributed element transmission lines to form the particular circuit; (2) implementing discrete lumped-element circuits; and (3) printing exposed microstrip lumped elements. Each of these techniques suffers from several disadvantages.
According to the first method (printing distributed transmission lines that form the individual components of the circuit) a certain length of metal line is printed onto a substrate. The transmission line is modeled as a continuous chain of capacitors, inductors and resistors (series and shunt) along the length of the transmission line. To provide a circuit element having particular electrical characteristics, transmission lines of varying lengths (which are computed as a function of frequency) and line-widths (which define the impedance) are networked into a specific geometry. This geometry is then printed onto a substrate to form the particular circuit element, or is etched into a substrate that has been metallized.
This first method suffers from several disadvantages. First, because the length of line required to model a particular element is dependant upon the wavelength of the signal being propagated, the technique is not useful at frequencies below about 1 GHz. As the propagation frequency decreases, the associated wavelength of the signal increases, and therefore the line lengths necessary to provide the needed functional elements become too large. This increases the size of the resultant circuit for most applications, and is impractical for space communication satellites. In addition, if the desired operating bandwidths are large, the size of the circuit increases even further since the number of network sections increases. For space-based communication applications, where operating bandwidths of several hundred MHz to several GHz are common, and where space and weight are at a premium, this technique is undesirable.
The second method (providing discrete lumped-element circuits) does not involve printing elements onto a substrate. This technique requires hand soldering and assembly of individual discrete lumped elements onto the signal carrying substrate. If a particular circuit is required to use certain passive components, then these components - capacitors, inductors, resistors, etc.--are procured and are directly assembled onto the substrate using surface-mount technology, just like any conventional printed circuit board.
This technique suffers from many disadvantages, such as: (1) it requires the procurement of individual components that must be assembled onto the substrate, therefore requiring time consuming and expensive handling and manufacturing procedures; (2) for space-based communication applications, it requires the procurement of expensive mil-spec or space-qualified components that may have to be tested in-house before integration into the substrate; (3) it requires human intervention to assemble the discrete components, inevitably leading to lower quality and enhanced defects; and (4) circuits designed using discrete components are generally limited to an upper frequency of 1 GHz, due to the physical size of the discrete components that become comparable to the wavelength of the signal frequency, resulting in distortion of their individual characteristics.
The third method is printing microstripline lumped elements. An example of a microstripline lumped element circuit is shown in U.S. Pat. No. 5,489,880 (the "'880 patent") to Swarup, assigned to the assignee of the present invention. The teaching of this commonly owned patent is incorporated herein by reference. As seen in the '880 patent, a microstripline architecture is an open structure in which a conductor pattern is printed or etched on top of a substrate. Lumped elements, such as interdigital capacitors and inductors are printed in metallized form onto the substrate layer. The microstripline approach is very common in Monolithic Microwave Integrated Circuits (MMIC.) This structure consists of a ground-substrate-signal architecture with the signal layer exposed to the atmosphere above the substrate. The substrate material is typically an expensive GaAs substrate.
The primary disadvantage of the microstripline approach, particularly for space-based communication applications, is electro-magnetic radiation from the circuit. Because the microstripline architecture is open on the top, where the high frequency signals are being propagated, such a structure can radiate these signals into the space above the substrate, potentially coupling the signals from the substrate to physically adjacent components and circuits. This creates cross-talk between the microstripline circuit and adjacent circuits, which degrades the functionality of the satellite payload. Therefore, it is undesirable to package other circuits or elements close to the top of the microstripline substrate, since signal noise may be radiated onto these other elements. This is a serious disadvantage of the microstripline approach in satellite applications, where size and packaging are key constraints. Another disadvantage of this approach is the use of expensive GaAs substrate materials, and the fact that such designs are only economical when large production quantities (thousands) are involved. Therefore, this technique is not cost effective for small numbers of units, which is typical in the satellite field where only a few number of specialized circuits may be required.
Therefore, there remains a need in this art for a lumped element stripline circuit having an architecture that suppresses radiation from the signal layer from coupling to adjacent circuits, and allows for integration of multifunctional elements, e.g., digital logic, IF, RF, Microwave, etc., within a single package.
There remains yet an additional need for such an architecture that can be used to implement various circuits, such as power splitters/combiners, solid state switch matrices, corporate feed networks, filters, couplers, etc.
There remains a need in this art for a method of making a lumped element stripline circuit having a ground-signal-ground architecture that suppresses radiation from the signal layer by sandwiching the signal layer between at least two ground layers.
There remains yet an additional need for such a method where numerous laminates are placed on either side of the signal carrying layer so as to define a stripline structure.
There remains a more general need for a method of making a lumped element stripline circuit that is low cost, is small in size, is easy to manufacture, does not radiate to adjacent circuits, is highly integrateable, and can be extended to many circuit implementations.
There remains a more particular need for a printed lumped element stripline packaging method for use with signal carrying circuits that can be cheaply and easily integrated into a satellite payload without concern that the circuit will create electromagnetic cross-talk to adjacent circuits.
There remains an additional need for such a packaging method that uses commercially available soft substrates that are laminated together to form a ground-signal-ground structure that prohibits radiation.
There remains a further need in this area for a printed lumped element circuit structure that can be used to implement specific passive circuits that operate from several hundred MHz up to several GHz.